I notice your cuts are a little ragged and torn. A sharp knife is a must when working with foam. Foam dulls blades extremely quick. Invest in a blade sharpener.
To smooth out rough edges, a dremel tool or sandpaper of varying grits can fix the problem.
For details you can either score in the details with a knife and then use a heat gun to open up the lines for the detail to show up or you can layer thinner foam (2mm or 6mm) or even better an combination of both.
Use my build of the S6C SOCOM as reference. To scoring details and layer build up.
If you have a self healing cutting mat then you can use that to sharpen your blade too. It's not better than a new knife, but it is definitely better than nothing. Just give it a few swipes on each side between cuts.
Also contact cement on both sides, wait five minutes, press together. That will create better looking seems than hotglue.
Barge is the most popular and well known. It can be pricey and not readily available everywhere.
Any other brand will work. Double check the instructions for set time. Some will require 5 mins of set time after application before putting parts together. Others may require 15mins of set time.
Talking about spray paint, good old Montana Cans come to mind. They have the broadest range of colours, i believe. And primers for all sorts of stuff. They have various "effect" collections (stone, metal, etc.) and are not too expensive. Marabu is the cheapest and sometimes surprisingly best, but i don`t know if they sell those outside Europe.... But if you want a custom made colour than you`ll have to switch to acrylics.
Okay, cool.! We do have/ had one...? I am not on facebook, so...don`t know if still up. You are encouraged to be active on this site by the staff, i am more comfortable with it than social media anyway.